1. Field of the Invention
The present invention relates to the design of semiconductor light-emitting devices. More specifically, the present invention relates to a technique for epitaxially growing p-type nitride semiconductor material and a method for fabricating semiconductor light-emitting devices using such p-type nitride semiconductor material.
2. Related Art
Group III-V nitride semiconductor materials, including compounds (e.g. GaN, InN, and AlN) and alloys (e.g. AlGaN, InGaN, and AlGAInN), are widely used in the manufacturing of short-wavelength light-emitting devices such as light-emitting diodes and laser diodes, as well as the manufacturing of high frequency electronics. The demand for high-brightness light-emitting diodes (HB-LEDs) using group III-V compound semiconductor materials has grown significantly over the years. HB-LEDs have a wide range of uses in the photonic industry, solid state electronic devices, automotive lighting systems, and other applications.
A P-N junction is an essential structure in the fabricating of light-emitting devices. When forward-bias is applied to a light-emitting device, the carriers, namely holes from the p-type layer and electrons from the n-type layer, recombine in the P-N junction region and thus energy is released in the form of photons. An active region formed by a multi-quantum well (MQW) structure between the p-type layer and the n-type layer facilitates a higher carrier density and hence an increased recombining rate of the carriers, which can improve light-emitting efficiency.
Techniques for epitaxially growing an LED structure with group III-V nitride materials include metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxial (MBE), and Hydride Vapor Phase Epitaxy (HVPE). Substrate materials used for epitaxial growth include sapphire (AL2O3), silicon, and silicon carbide (SiC).
When Si and magnesium (Mg) are used as the donor and acceptor dopants respectively for fabricating group III-V nitride materials, it is relatively easy to obtain high carrier density in n-type nitride materials. However, this is not the case for p-type nitride materials.
During the fabrication of a p-type nitride material, hydrogen (H2) is often used as the carrier gas to increase the density of the acceptor, which is often Mg. However, H2 and Mg can form an electrically inactive Mg—H complex, which reduces doping efficiency. As a result, a Mg-doped p-type nitride layer might have fewer activated acceptors than one doped with other materials.
To overcome the problem described above and to obtain a low resistivity p-type nitride layer, low-energy electron beam irradiation (LEEBI) and/or an annealing treatment in a H2-free environment can be employed after the growth of the p-type nitride layer. These additional processes break down the Mg—H complex and electrically activate the acceptor. However, the p-type nitride layer has to be relatively thin so that these additional processes can be effective.
On the other hand, having a relatively thick layer of p-type nitride material can improve the quality and reliability of the LED. Growing a high-quality, thick p-type nitride layer often requires a high temperature environment and growth for a prolonged period of time. However, such prolonged high-temperature growth can damage the adjacent quantum-well active region and thus reduce the efficacy of the carrier activation process. Consequently, the number of activated acceptors decreases, and so does the efficiency of the light-emitting device.
Hence, what is needed is a method for growing a relatively thick p-type nitride layer that has a high carrier density without compromising the quality of an adjacent MQW region.